Class BridgeEventCallbackArgument is a wrapper on a callback
arg plus the host and port of the client originating the event containing
this BridgeEventCallbackArgument. It is created by the server
when it receives an update request from a client and used to determine the
originator of the event by the CacheClientNotifier. The original
callback argument is wrapped by this one and replaced in the event sent to
CacheListener, CacheWriter and CacheLoader.